Digester system



Nov. 10, 1931.

W. H. SWANSON ET AL DIGESTER SYSTEM Filed March 6. 1931 2 Sheets-Sheet l Nov. 10, 1931. w. H. SWANSON ET AL DIGESTER SYSTEM Filed March 6. 1931 2 Sheets-Sheet 2 Patented Nov. 10, 1931 UNITED STATES PATENT OFFICE WALTER H. SWANSON, OF MENASHA, AND LLOYD LANG, OF KIMBERLY, WISCONSIN, AS- SIGNOBS '10 PAPER PATENTS COMPANY, OF NEENAH, WISOON SIN, A CORPORATION OI WISCONSIN DIG-ESTER SYSTEM Our invention relates broadly to the digestion of wood pulp forthe manufacture of paper and the like and has to do more particularly with improved apparatus and method for the circulation of digesting liquor and the maintenance of substantially uniform temperatures thereof at all points in the digester. Our invention contemplates the use of an auxiliary heater for the liquor outside of the digester', means for continuously circulating the hot liquor through the heater and digester, and means for automatically maintaining the temperature thereof.

According to the ordinary system for operating digesters, there may exist widely variant temperatures at various points in the digester at any given time. Inasmuch as temperature is the most important factor governing the quality of sulphite pulp, it is obvious that lack of temperature control can'result only in extreme non-uniformity in the character of the pulp produced in any cook. Thus, portions may be over-cooked, other portions properly cooked and still other portions under-cooked. Furthermore, uniformity of the product from one cook to another 'is diflicult to maintain inasmuch as these temperature variations are. not reproducible in consecutive cooks.

A principal object of our invention, therefore. is to eliminate the conditions above referred to and to substitute temperature control whereby the liquor will he of substantially uniform temperature at all points in the digester at any time, and the quality of the product will be substantially uniform from one cook to another, inasmuch as the temperature conditions may be reproduced at will.

Another ohjectof our invention is to provide a new method and,apparatus for ac complishing the results outlined above.

The principal results "which are achieved by our invention are: i

and

1. Uniformity of pulp quality within an individual cook.

2. Uniformity of pulp quality fromcook to cook because of ease of repetition of cooking conditions. This latter is made still more possible because automatic means for controlling temperature can'be utilized, as will be shown below. This naturally eliminates the human element in cooking.

3. Improved quality of pulp because of elimination of overcooked and undercooked portions of the pulp, which have a deleterione effect on the finished pulp taken as a plete'filling of the digester can be employed with the normal cooking system, but in general pulp quality suflers and operating difficulties arise through this practice. These are not experienced with the new system.

Turning now to the drawings illustrating I preferred apparatus for carrying out our invention T Fig. 1 shows an elevational view partly in section of a digester, heat exchanger, and auxiliary parts comprising our improved sys tem.

- Fig. 2 is a section taken on line 22 of I Figure 1. p

Fig. 3 is a view, artlyiin el vation and partly in section, 0 the\ pro ed heater;

Fig. 4 is a section taken on line 4-4 of Figure 3.

The numeral 10 represents a digester which 7 comprises a cylindrical portion 11, an inverted cone-shaped base portion 12, and a substantially spherical dome portion 13. A

removable air tight top closure 14 .of any suitable type is provided. The digester is formed with an outer metallic casing 16 and an inner llmng 17 of acid-proof ceramlc material or the like.

fourth inch holes. Such a design is desir able for the following reasons In this form, the strainers cover a large portion of the digester content, resulting in uniform removal of liquor from all parts and thus reducing channelingtendencies. Furthermore, uslng a large number of stralner holes causes the linear velocityof the liquor passing through any one hole, to be very low. Hence, there is no tendency for the liquor to draw small chips or fibers into these holes and clogging them. As shown in Figure 2, the strainer may be in two parts.

Disposed in the strainer 20 is a pair of T fittings 21, which may be oppositely disposed on the circumference and which communicate with pipe sections 22. The latter sections are connected in turn to elbows which, by means of suitable fittings, extend through the wall of the digester. It will thus be seen that the strainer 20 is supported by means of the oppositely disposed vertical pipe sections 22 I (only one of which is shown in Figure 1).

ating the balance valve.

The elbows '25 communicate by means of various other pipe fittings, which need not be described here, with a pipe 28, which is connected by means of an elbow 29 to a vertical pipe 30, which leads to a pump 31, a valve 31' being 1 disposed inpipe 30. The pump 31 need not be described here in detail.

The pump 31 discharges through a valve 32 to a heat exchanger indicated generally at 33. Condensed steam is removed from the heat exchanger by means of a liquid level controller attached to a balanced valve 7 6 in condensate line 78 from the heat exchanger 33. This liquid level controller permits of continuous removal of the condensate by actu- The condensate, being pure water and containing considerable heat, is returned to the boiler house to be used as make-up water, which effects appreciable economy in steam cost.

The liquor is forced by the pump 31 upwardly through the heat exchanger and thence by suitable fittings through a down pipe 35. This pipe is connected to an elbow 36, nipple 37, valve 38 and T 39, thence to a pipe section 40, elbow 41 and T 42. The T fitting 42 is connected to a pipe section 43 which in turn is connected to an elbow 41 which leads upwardly through a. valve 45 to a This strainerbilled s0 1.25

ly into the digester and have their outlets at points spaced inwardly from the wall ofthe cylindrical portion 12 thereof. In the preferred embodiment six nozzles 50, equally spaced about the periphery of the digester. are provided.

In order. to insure digestion of chips in the extreme lower portion of the cone 13 of the digester. a by-pass is provided comprising a pipe 51 tapping the main feed pipe 43, having a valve 52 therein and connected by means of a. nipple 53 to the bottom T 55 of the digester. It will thusbe seen that each increment of liquor is forced by displacement upwardthrough the chip mass and is substantially evenly distributed from the header, thus providing substantially equal temperatures througl'iout the digester.

\Vhile thedimensions of the'digester may vary within wide limits, the one under consideration has a maximum diameter of 14 feet and is about 35 feet in length from the. bottom of the cone to the top of the domeand has a volume inside of the lining of about 3051 cubic feet. The capacity of the digester in pulp is approximately 5.75 tons of spruce, 5.75 tons of balsam and 5.25 tons of hemlock. In carrying out the process embodying our invention, the 'digester is filled with wood chips of the usual form and with cooking acid. The entering acid may vary in temperature from 30 degrees C. to 50 degrees (1, and is'made up of a solution of bisulphite salt, usually a bisulphite of calcium or dolo-' mitic lime, and free sulphur dioxide in excess. A common analysis is as follows:

' Total S0 5.5%; free S0 4.25%; com- (computed as monosulphite).

Having completely filled the digester with both chips and acid, it is prepared for cooking by bolting on the top cover and closing all valves. The valves 31, 32 and 38 are then opened and the pump 31 is started, whereupon the liquor is caused to circulate.

Considering at this point the main circulatory system of the liquor, it will be seen that the liquor in the upper portion of the digester flows into the annular pipe 20 and thence downwardly through the pipes 22, 28 and30 to the pump 31 whence it is forced up through the heat exchanger 33, where it absorbs heat. The liquor is then forced down through the pipe 35 through the various fittings 36, 37. as. 39, 40, 41, 42, 43, 44 and 45 saturated withthe acid liquor, which also gives up some of its heat to the chip mass, thus raising its temperature to the cooking range, causing the digestion reaction topro- 'ceed so as to disintegrate the wood into its ultimate fibers.

It is a well known fact that wood, particularly when quite dry, containing 40% moisture or less, will absorb a considerable quanwhereby a portion of the heated liquor com tity of acid. Inasmuch as the system just described involves heating of the acid liquor by indirect means (in contra-distinction to the usual system wherein steam is introduced directly into the digester contents and con denses there) this results in a lowering of the acid level in the digester when using dry wood. It sometimes drops to a point wherein a considerable portion of the chipped mass in the top dome of the digester becomes uncovered by liquor. This is undesirable for the reason that these chips heated out of contact with the acid are likely to become brown or charred, resulting in the rejection of an appreciable portion of the digester content as screenings or tailings; also some of these chipsbreak up in the process of refining. causing brown specks or shives in the resultant pulp and thus lowering its quality.

In order to avoid this situation, we provide certain optional features which comprise a valve 58 connnunicating with the T fitting 39,

ing from the heat exchanger may be diverted. The valve 58 communicates with a continuous heater represented at 59, which receives live steam from a steam pipe, 60 con rolled by means of a valve 61. The liquor is here intimately mixed. in the heater 59 with the live steamwhich is condensed therein, resulting This bypassed liquor rejoins the liquor of thelnain line in the T fitting 42.

In cooking wood for pulp production a predetermined rate of temperature increase of thedigester content is required, which will give the pulp the desired quality in the time allotted for cooking. In our process for cooking the preferred time is from 9. to 10 hours. We have found that a temperature schedule somewhat as follows for the digester I content, yields a pulp of the desired quality.

1. A rate of increase of temperature of 40 C. per hour from the starting temperature,

which is usually around degrees centigrade, to 110 degrees centigrade'. This requires a one and one-half hour time interval for this preliminary heating period. 2. .A rate of increase of temperature of ',7 degrees per hour for the next six hours from 110 degrees C. to 152 degrees C. main cooking period. 3. A holding period of two hours whe the digester contents are maintained at the.

maximum temperature of 152 degrees (I This is the finishing period of the pulping This is the" process. This schedule makes a cooking time of nine and one-half hours and contains two periods when the temperature rises at different rates-and one when it is maintained constant.

e accordingly provide the following means for producing the temperature schedule outlined above. 7

A thermometer bulb 63 of the vapor tension type is inserted in the cross portion of the T fitting 42. This thermometer communicates by means of flexible tubing 64 with a conventional time-temperature control 65 which is preferably disposed on the digester charging floor (not shown). A compressed air inlet 66 leads through a valve mechanism in the controller (which is actuated by the vapor tension in thermometer 63) to a motor diaphragm steam valve 68 disposed in the referred to are all more or less conventional and hence are not shown in detail.) Also, attached to the air valvejust referred to is the vapor tension thermometer 63. The cam is so designed so that at any desired point in the cook the air is actuated by the cam so as to permi" the flow of air to the motor diaphragm valve. whereby the opening of the latter is adjustedto permit steam to pass through it and into the heat exchanger at such a rate that the liquor passing through the heat exchanger is heated to the desiredtemp'erature. The thermometer is an important feature inasmuch asit measuresthe temperature of the liquor. and if it is higher -or lower than that desired,-it automatically constant in the steam main 69 bv means of a pressure reducing valve (not shown) of a conventional or other suitable tvpe. The

steam pressureis preferably held atabout 80'lbs. gauge, which gives a saturated steam temperature of about 323 F.

Obviously. other temperature schedules than that given above may be used and the time temperature controller may be caused to function according to any desired -sched-.

ule by providing a cam of proper design. Another method of temperature control which we have successfully employed is as follows:

A pressure regulating valve is attached to the heat exchanger and actuates a balanced drical portion 11 of the digester is a small,

til

valve in the steam main. For any given rate of temperature rise of the liquor, a predetermined steam pressure is maintained on the steam side of the heat exchanger by setting the valve accordingly. To change the rate of temperature rise, the regulating valve 58 is adjusted. While this system works satisfactorily, it requires more'attention from the operators than is desirable.

Pressure control release the-excess gases in order to maintaina predetermined minimum. In the preferred system embodying our-invention, the maximum pressure is usually held at between to lbs. gauge. In order to control this, an automatic system is used, as follows: I

Mounted in the upper wall of the cylinoutlet fitting 80, which may he referred to as a pressure plug. This plug is connected by means of tubing 81 to release controlling mechanism 82 and by means of a tube 83 to a pressure recorder 84. Inasmuch as the re corder 84 records the pressure and does not enter into its control, it may be neglected in the description which follows.

A pressure pipe 81 is connected to an air valve (not shown) disposed in the controller 82, which valve controls the flow of compressed air through the instrument from the tubing 66 to the line 86, the latter being connected to'a diaphragm motor valve 87 and a relief line 88 which communicates with the upper portion of the digester through. the conduit 89 in which is a valve 90.

It will be noted that --the conduit 89 is tapped into the closure member 14, the latter seating a strainer 91 which serves to prevent the passage of-pulp out of the digester into valve in the relief line 89. When the maximum pressure is attained. the air valve in the controller is automatically opened, permitting the compressed air to flowthrough it and the line 86 into the motor diaphragm valve 87. The compressed air opens this valve and permits gases in the digester'to pass out into the line 88 to the reclaimer tank (not shown), where they are recovered forfurther use. The venting of the gasesfrom the 'digester causes the pressure therein to drop, which drop in pressure is transmitted to the controller which actuates the air s u ply to the motor diaphragm valve, closing t e' latter. This action continues so long as the setting of the controller is not changed- It is customary in the practice of sulphite.

pulping preparation to materially reduce the pressure of the digester contents near the end of the cook, in order to recover the excess of sulphur dioxide which remains in the cooking acid after the pulping process is completed. To accomplish this recovery by means of our pressure controlling means just described, it is only necessary to set the controller at a new and lower pressure point, say about 50' pounds gauge. This resetting permi s the diaphragm motor valve to open wide and remain so until the gases vented from the digester have been suflicient to drop its pressure to the new setting. At this point, the 4 controller again becomes operative, tending to maintain thepressure there. However, the digester contents are customarily blown out at this time.

Heat emckange'r' We consider the heat exchanger an important part of our improved system, inasmuch Figures 3 and 4 which. show the heat exchanger 33 in detail, this device comprises a pipe member 100,' which may be secured as by screw-threading or the like to an annular collar 101. This collar may be secured as by bolts 102 or the like to a similar pipe'section 103, the latter having an annular flange 101 through which the bolts extend, an apertured disk-like header 105 being interposed between the flange 104 and the collar 101. The pipelOOmaybetapped at a plurality of places adjacent its lower portion to provide openings for a condensate outlet 106 and for pipes 107 and 107, communicating with the liquid level controller 75. The pipe 100 may be screw-threaded or otherwise secured to a. collar 108, which maybe secured as by bolts 109 or.the like to an annular flange 110 of a T- fitting 111, which communicates with-the steam line as by means of a nipple 112 or the like. The T 111 is flanged at 113 and is thus secured by bolts 111'or the like to a collar 115, a perforated header disk 116 being secured between the collar and the flange 113. A plurality of tubes 118 are secured in the headers 105 and 116 in the usual manner and serve to convey liquor through the heater, it being'understood that-the steam which enters the heater through the nipple 112 circulates around the tubes and is drained off The particular design of the heat exchanger is considered important. The main features thereof are as follows: The number of tubes was chosen at 22 because with the size shown, 1" diameter, this number gives. an area suflicient to permit of a linear ve locity of the liquor through them of 10 feet per second. This velocity results when a rate of circulation of liquor through the system of 550 gallons per minute is used. Preliminary experiments in a small scale, heater showed that there was no appreciable tendency for the surface of the tubes to scale with gypsum or calcium and magnesium sulphite from the acid at velocities above 8. feet per second; therefore, a velocity of 10 feet per second was chosen for the preferred installation and the heater designed accordingly. This is an important factor in the indirect heating of sulphite cooking acid successfully. Further, the material in the tubes consists of 18 percent chromium8 per cent nickel alloy iron, commonly called KA This was used not only because of its resistance to corrosion by sulphite acid, but also because it had very little tendency to scale when liquor velocity past-its surface is 8' or more per second,'as before pointed out.

.Coppcr, the other. commonly used metal in sulphite practice, scales rapidly, even when the acid is velocity.

The length of the tubes was set at 18 5%" because with 22 tubes 1" in diameter of this length, sufiicient heat-transferring surface is presented between the steam and the circulating liquor to permit of heating the digester contents at the fastest rate'usually employed in cooking practice; namely, 50 C. per hour. The pump is placed at a low point to permit of the liquor being forced in at the bottom of the heat exchanger, where the steam is at its lowest temperature, and passing out at the top, where the live steam is introduced.

The continuous heater 59, used in this installation for introduction of direct steam into the acid .when necessary, should beof the injector type. Steam might be introduced directly into the acid line, but this 'causetsflsuch 1:: ViOIGiIIIt hammer that it is impossi e to eep t e pipin ti ht. It also might-be introduced directly intg the digester in the conventional manner, but this'leads to local overheating and channeling of the acid, which is the factor which this system is designed to overcome. Therefore, the principle'followed in this whole scheme has been to heat the liquor externally and then pass it through the digester.

It will be noted that in our improved system the liquor circulates fromthe bottom to the top of the digester rather than from the top tothe bottom, as .is the usual practice.

passed over its surface at high This method of circulation was chosen be cause of the nature of the liquor. As pre viously stated, the liquor is a solution ofbi- 'sulphite and sulphur dioxide in Water. The

solubility of the sulphur dioxide in water depends at any particular temperature on the pressure at which the solution is maintained. The higher the pressure, the higher will be the concentration of free sulphur dioxide in the solution. At the top of the digester, the solution is undera pressure of pounds, for

example, during the major portion of the cook. At the bottom, it is at this pressure plus the hydrostatic pressure due to the weight of the liquid in the digester, or approximately pounds total pressure inour preferred installation. Now, if theacid were removed from the bottom of the digester, passed through the heater and released at the top, it would have passed from a condition of one temperature and pressure to anew condition of a lower pressure but a higher temperature, due to its being heated. This would cause some of the dissolved gas tomaintenance of a high sulphur dioxide content.

A multiplicity of entry points for the acidseven in this s stemis in some cases,

of some advantage in order to obtain even -d1str 1but1on of the cn'culatmg acid over the whole of the digester contents. It seems desirable to have the temperature of the digester contents uniform across any horizontal plane. Thisis the objective-sought in introducing the liquor as shown. However, such temperature uniformity may be achieved by admitting the acid at a single point, if

the digester is uniformly packed with chips.

The fact that digester temperatures can maintained uniform by this system makes I possible the application of an automatic timetemperature instrument for controlling the temperature duringthe cooking cycle. The advantage is obvious, namely, the elimination of the human element, with resultant uniformity of product from cook to cook.

The automatic pressure control device is used as a matter of conveni'ence lightening the task of the operator.

Another feature which has developed in the use of this system is the desirability of uniformly distributing the chips throughout the digester during the filling period. With already mentioned. Naturally, this means a loss of productive capacity. Mechanical devices are on the market which distribute chips uniformly throughout the digester durin its filling, eliminating the-coning effect am increasing the quantity of chips charred into the digester. The uniformity of distribution is very desirable in that it eliminates uneven pressures on the chip mass and consequently is a very material aid in preventing channelmg of the circulating liquor. It is a very de-.

sirable feature to incorporate in this system. These chip distributors have not been particularly successful with the conventional system of cooking because they lead to undercooking in the lower portions of the digester, with conse uent hangmg up of this raw stock when the igester contents are blown out. This difliculty is not experienced when our improved circulatin system is employed for cooking and a chip ing device is used.

Throughout the installation we prefer to use alloy steel containing about '18 per cent chromium and 8 per cent nickel, commonly called KA in all pipings and fittings. which come in contact with the cooking liquor. This material is used because of its veryhigh corrosive resistance and is highly preferable to copper and .bronze.

. Various modifications, variationsv and improvements coming within the spirit of our invention will doubtless occur to those skilled in the art. Hence, we do not wish to be limited to the specific form shown or uses mentioned except as set forth. in the appended claims, which are to be interpreted as broadlyas the state of the art will permit.

We claim as our invention:

or the like,

1. A process of digesting wood comprismg introducing digesting liquor in a plurality of jets into the lower portion of a digester containing pulp, removing said liquor from the upper portion thereof, heating said liquor, and returning it to the lower portion of said digester to repeat the cycle. 2. -A process of digesting wood or the like,

comprising'introducing dlgesting liquor in a plurality of jets into the lower portion of a digester containing pulp, removing said liquor from the upper portion thereof, passing said liquor upwardly through an indirect heater, and thence conducting said liquor to the lower portionof said digester for repeating the cycle.

3. In a system for digesting wood or the like, a digester, means for jettingliquor into the lower portion of said digester, means for withdrawing liquor from-adjacent the upthe lower, portion of said digester, said means comprising a plurality of upwardly extending nozzles, means for withdrawing liquor from' adjacent the upper ortion thereof, a vertical indirect heat exc anger, means for conducting liquor to the lower portion of said heat exchanger, means for withdrawing liquor from the upper portion thereof and returning itto the lower portion of said digester for recirculation, and

means for circulating a heating fluid through said heat exchanger.

5. In a system for digesting wood or the like, a digester, means for jetting liquor into the 'lower portion of said digester, said means comprisinga'plurality of upwardly extending nozzles, means for withdrawing liquor from adjacent the upper rtion thereof, a vertical indirect heat exc anger,

means for conducting liquor to the lower portion of said heatexchanger, means for withdrawing liquor from the, upper portion thereof and returningit, to the lower por-v tion of said digester for recirculation, means for circulating a heating fluid through said heat exchanger, and means adjacent the li uor inlet of said heat exchanged forcircu ating saidliquor.

6. In combination with a liquor circulating system, a heater, means for introducing heating fluid into said heater, means controlling said fluid introducing means for maintaining. a predetermined time-tempera ture schedule, and means controlled by the temperature of liquor in said system and acting as a check on said time-temperature controller, whereby the temperature of liquor in said system is automatically maintained within predetermined limits.

7. In a digester system, a container for wood or the like, an indirect heater for digesting liquor, means for circulating liquor from said digester to said heater and back to said digester, and means for automatically controlling the flow of heating fluid'to'said heater according to a temperature schedule.

8. In combination with a system as defined in claim 7, temperature controlled means disposed in said system adjacent the inlet of liquor to said digester for maintainpredetermined timean indirect 10. In a process of digestin wood or the like, the method of circulating digesting liquorthrou h a system including a dlgester, heater, and conduits connecting said digester and heater, and maintaining a substantially constant level of liquor in said digester b introducing heated fluid into a portion 0 digester.

11. In a digester stem, a container for wood pulp or the li e, an indirect heater, means for circulating digesting liquor from the liquor line outside of said said digester to said heater and back to said I digester, and means formaintaining a substantially constant level of liquor in said diester, said means comprising an injector for heated fluid disposed in the liquor line outside of said digester.

12. In a digester stem, a container for wood pulp or the li e, an indirect heater,

means for circulatingx-digesting liquor from 7 said digester to said eater and back to said digester, means for maintaining a substantially constant level of liquor in said dige'ster,

said last-named means comprising an injector for heated fluid disposed 1n the liquor line outside of said digester, a valve controllin the flow of heating fluid to said heater, an

meanscontrollewby thetemperature ofliquor adjacent the li uor inlet of said digester for controlling sai valve and thus maintaining the temperature of said liquor within predetermined limits.

13. A process of digesting cellulosic material, comprising cookin said fibers in the digesting liquor acco' ing to a schedule wherein the temperature is raised approximately 40 C. per hour from about C. to about 110 0., then increasing the temperature at approximately the rate of 7 0. per

hour until a temperature of about 152 C. is

attained, and holding the temperature of about 152 C. for a period of about two hours.

14. A process of digesting cellulosic-fibers wherein there-is used a digesting liquor containing a gaseous digesting agent in solution, comprising introducing liquor into the lower portion of a digcster, withdrawing the liquor adjacent the upper portion thereof, and conveying the same to the lower portion of an indirect heat exchanger, withdraw the liquor from the upper portion of said heat exchanger, and returning it to the lower portion of said digester for re-circulation.

15. A system as defined in claim 6, wherein said'means which acts a ls-2a check on the,

time-temperature controller is controlled by the temperature of liquor adjacent the liquor inlet of said digester.

WALTER H. SWANSON. LLOYD LANG. 

